This invention relates to satellite communications systems, and particularly to coupling arrangements using a zero dB hybrid or directional coupler.
An important aspect of modern business relies upon inter and intra-continental communications, large amounts of communications traffic are carried by communication satellites. Many such satellites are in use, and new satellites are currently fabricated for new applications and for replacement purposes. The fabrication and launch of a communications satellite tend to be capital-intensive, and improvements which increase the reliability and life of a spacecraft, improve its performance or reduce its cost, are desirable.
FIG. 1 illustrates a simplified communications satellite 10 orbiting about the earth 8. Satellite 10 includes a body 12, a pair of solar panels 14a and 14b for powering the spacecraft, and a transmit-receive communications antenna 16. Antenna 16 receives signals from one or more earth stations, processes the signals and repeats the information, often at a different carrier frequency, back toward the same and/or other earth stations. Identical reference labels in different drawings reflect identical elements earlier described.
FIG. 2a illustrates, in simplified block diagram form, a communication system which may be used in conjunction with satellite 10. In FIG. 2a, an antenna illustrated as 216a represents a portion of the receiving section of antenna 16 of FIG. 1. For example, antenna 216a of FIG. 2a may represent a vertically-polarized (as opposed to horizontally-polarized) receiving portion of antenna 16. The signals received by antenna 16 of FIG. 1 may include a plurality of information channels in adjacent frequency bands extending over a cumulative frequency band such as 13.5 to 14.0 GH.sub.z. Each individual channel may have a bandwidth, for example, of 6 MH.sub.z, which might be sufficient to carry a standard television channel or a plurality of multiplexed telephone or data subchannels. Each channel can be separated from the channels on adjacent frequencies by frequency filtration. In order to reduce channel interaction, each channel, as transmitted to antenna 16, is at a polarization orthogonal to that of the adjacent-frequency channels.
Antenna 216a couples signals received with a vertical polarization to a receiver 212, which may include, for example, a bandpass filter (BPF) 214 covering the cumulative bandwidth, a low noise amplifier (LNA) 216, and a frequency converter including a mixer 218 fed with local oscillator (LO) signals from a source (not illustrated). The received signals are applied from receiver 212 to a demultiplexer illustrated as a block 220. The frequency-converted signals at the input of demultiplexer 220 include a plurality of semi-adjacent channels, since the horizontally-polarized adjacent channels are discriminated against by vertically polarized antenna 216a. The cumulative bandwidth of the converted signals may be, for example, 11.7 to 12.2 GH.sub.2, and within that bandwidth, a plurality of channel spectra may be included, centered at frequencies designated as f.sub.1, f.sub.2, f.sub.3, f.sub.4 . . . in FIG. 2b. They are not designated f.sub.1, f.sub.3, f.sub.5 . . . , because the adjacent horizontally polarized signal channels are ignored in relation to the discussion of FIG. 2a. While the down-converted signals produced by receiver 212 could in principle be amplified together, by a broadband amplifier, before transmission back to the earth, the nonlinearities of amplifiers are such that intermodulation distortion might degrade the signals at the desired output signal amplitudes (levels). In order to amplify the signals to the desired level without intermodulation distortion, they are separated into individual channels for amplification by individual amplifiers. Distortion occurs in the individual amplifiers, but may be manifested more as a compression, which can be ameliorated by a predistortion equalizer (not illustrated) in each channel.
Demultiplexer 220 filters the signals into separate channels in accordance with frequency. For example, signals about "odd" frequency f.sub.1 of FIG. 2b are coupled into a channel F.sub.1, signals at "even" frequency f.sub.2 are coupled, into a channel F.sub.2, . . . , signals at even frequency f.sub.2N are coupled into channel F.sub.2N, and signals at odd frequency f.sub.2N+1 are coupled into channel F.sub.2N+1. A plurality of amplifiers 222a, 222b, 222c . . . 222d, 222e are associated with output channels F.sub.1, F.sub.2, F.sub.3 . . . F.sub.2N, F.sub.2N+1, respectively, of demultiplexer 220. As is well known to those skilled in the art, a redundancy scheme (not illustrated) may be used for substitution of spare amplifiers in the event of a failure, or for using remaining amplifiers for higher priority uses rather than lower priority uses, as described in U.S. patent application Ser. No. 07/772,207, entitled "Multichannel Communication System with an Amplifier in each Channel," filed on or about Oct. 7, 1991 in the name of H. J. Wolkstein.
The separately-amplified signals in each channel F.sub.n of FIG. 2a must be re-multiplexed by combining in order to allow transmission by a single antenna arrangement. Just as the effective skirt selectivity or channel isolation of demultiplexer 220 is improved by applying only semi-adjacent channels for demultiplexing into channels F.sub.1 -F.sub.2N+1 (where the hyphen represents the word "through"), the multiplexing of the vertical channels F.sub.1 -F.sub.2N+1 is improved if the channels to be multiplexed are separated in frequency as much as possible. Thus, for improved skirt selectivity, channels F.sub.1 -F.sub.2N+1 are recombined or multiplexed by a pair of multiplexers 224E (even), 224O (odd). Odd channels (also called "odd-mode" channels) F.sub.1, F.sub.3 . . . F.sub.2N+1 are applied to multiplexer 224O, and even channels F.sub.2, F.sub.4 . . . F.sub.2N ("even-mode") are applied to multiplexer 224e. Each multiplexer 224 combines the signals received from its respective channels onto one of two combined transmission paths 226O and 226E.
If the multiplexed signals from all the channels were available on a single transmission path rather than on transmission path pair 226O, 226E, the signals could be applied to the transmit antenna (represented by feedhorns 216B, 216C, 216D and 216E) by way of a power divider or coupler having a single input port. Feedhorns such as 216b-216e may be used, as known, in conjunction with a reflector in order to aid in directing beam portions over a desired area, such as a continental area. However, since the signal to be transmitted is generated, as described, on two separate transmission lines 226O and 226E in order to provide increased filter skirt selectivity in multiplexers 224, a "two" port coupling arrangement to the transmitting antenna arrangement must be provided. The two-input-port feature is provided by a coupling arrangement illustrated as a block 228 in FIG. 2a. The odd channel signals on path 226O are applied to an input port 1 of block 228, and the even channels on path 226E are applied to an input port 2. Details of coupling arrangement 228 are illustrated in FIG. 2c.
FIG. 2c is a simplified block diagram of coupling arrangement 228 of FIG. 2a, and FIG. 3 represents a physical structure corresponding to that of FIG. 2c. Elements of FIGS. 2c and 3 corresponding to those of FIG. 2a are designated by the same reference numerals. Ideally, the odd- and even-mode signals applied to input ports 1 and 2, respectively, of coupling arrangement block 228 would be applied with equal phase to all of feedhorns 216b-216e. However, this ideal phase cannot be accomplished, for various reasons, including the difference in the frequencies passing through each channel, in that odd transmission path 226O carries frequency f.sub.1 which is below frequency f.sub.2, and also, if the number of odd and even channels is equal, channel F.sub.2N+1 would not exist in which case, even channel 226E would carry frequency f.sub.2N, which is above f.sub.2N-1. As a result, an acceptable compromise has been found to be the application of signal to the feedhorns with monotonically changing phase shifts. The phase shifts are in mutually opposite direction for the two inputs. This results in a beam tilt, but the beam tilts are mutually opposite for the positive and negative phase shifts.
In FIG. 2c, input port 1 of coupling arrangement 228 is connected to a first input port 231.sup.I1 of a first 3dB, 90.degree. hybrid or directional coupler 231. Input port 2 of coupler 228 is connected to a second input port 231.sup.I2 of coupler 231. Those skilled in the art are familiar with 3dB, 90.degree. directional couplers or hybrids, and especially know that the 3dB and 90.degree. values are only nominal, and that the actual values may differ depending upon conditions such as frequency and impedance. A first output port 231.sup.01 of hybrid 231 is coupled by a transmission path 244 to a first input port 232.sup.I1 of a second 3dB, 90.degree. coupler 232. Second input port 232.sup.I2 of coupler 232 is terminated, as known, in a characteristic impedance, as illustrated by a resistor symbol. A second output port 231.sup.02 of coupler 231 is connected by a path 246 to a first input port 233.sup.I1 of another 3dB, 90.degree. coupler 233. A second input port 233.sup.I2 of coupler 233 is terminated. A first output port 232.sup.01 of coupler 232 is connected by way of a transmission path 248 and a phase shifter 242 to first horn antenna 216b, which is part of antenna 16 of FIG. 2a. A second output port 232.sup.02 of coupler 232 is coupled by a path 250 to a first input port 234.sup.I1 of a fourth 3dB, 90.degree. coupler 234. A first output port 233.sup.01 of coupler 233 is coupled by a path 252 to a second input port 234.sup.I2 of coupler 234. A second output port 233.sup.02 of coupler 233 is coupled by way of transmission path 254 and a phase shifter 246 to horn 216d.
A first output port 234.sup.01 of hybrid coupler 234 of FIG. 2c is coupled by way of a transmission path 256 and a phase shifter 244 to horn antenna 216c. Second output port 234.sup.02 of coupler 234 is coupled by path 258 to phase shifter 248. The crossover of inputs to phase shifters 246 and 248 is provided as described in more detail below in order to maintain a constant phase progression at the outputs of horns 216b-216e.
As mentioned above, a monotonic phase progression across the feed horn apertures is desired. This monotonic progression may result in a slight beam tilt (squint). As illustrated in the simplified arrangement of FIGS. 2a and 2c, four feed horns are involved, and the total phase progression across the four horns is 135.degree.. A phase progression as large as 135.degree. causes a substantial beam tilt, but the actual beam tilts may be smaller, because the horn-to-horn phase progression can be decreased by causing the illustrated phase change to occur across a number of horns larger than four. However, the use of four horns is sufficient to explain the invention.
In operation of the arrangement of FIG. 2c, the odd-mode signals applied to input port 1 of coupling arrangement 228 are applied to input port 231.sup.I1. One-half the signal power (-3dB or 0.707 amplitude) applied to input port 231.sup.I1 is coupled to output port 231.sup.01 with reference (/0.degree.) phase, and the other half of the signal power is coupled to output port 231.sup.02 with a nominal 90 degree (/-90.degree.) phase delay. The signal at output ports 231.sup.01 and 231.sup.02 of coupler 231 may be written as 0.707/0.degree. and 0.707/-90.degree., respectively. Similarly, the even-mode channels applied to input port 2 of coupler arrangement 228 are applied to input port 231.sup.I2 of coupler 231, and are coupled, in equal amplitudes, to output port 231.sup.02 as 0.707/0.degree., and to output port 231.sup.01 with minus 90.degree. phase (0.707/-90.degree.). Thus, the signals arriving at first input ports 232.sup.I1 and 233.sup.I1 of couplers 232 and 233, respectively, each include a plurality of interleaved half-power odd and even frequency signal components. In FIG. 2c, phases, relative to the odd signals applied to input port 1 of coupling arrangement 228 from which they originate, of the signals which are produced at the various output ports of the couplers of FIG. 2c, are designated adjacent to the respective output ports. Also, the phases, relative to the even signals applied to input port 2 of coupling arrangement 228 from which they originate, of the signals which are produced at the various output ports of the couplers, are designated, in parentheses, adjacent to the respective output ports.
The interleaved frequency components (0.707/0.degree. and 0.707/-90.degree.) applied to input port 232.sup.I1 of coupler 232 of FIG. 2c are coupled with equal amplitudes to its output ports 232.sup.01 and 232.sup.02 with 0.degree. and -90.degree. phase, respectively. The interleaved frequency components (0.707/0.degree. and 0.707/-90.degree. applied to input port 233.sup.I1 of coupler 233 are coupled with equal amplitudes and corresponding phases to output ports 233.sup.01 and 233.sup.02. In this case, the reference-phase signal exiting from output port 233.sup.01 of coupler 233 has the same phase as the input signal, namely -90.degree., while the signal exiting output port 233.sup.02 has an additional 90.degree. phase shift, for a total phase shift of 180.degree.. Coupler 234 couples the signals applied to its input ports 234.sup.I1 and 234.sup.I2 to its output ports 234.sup.01 and 234.sup.02. The odd-mode signals originally coupled to input port 1 of coupling arrangement 228 are coupled to output ports 234.sup.01 and 234.sup.02 in equal amounts, whereby output port 234.sup.01 receives a first component at -90.degree. from input port 234.sup.I1, and a second component of -90.degree. from input port 234.sup.I2, which is phase shifted within coupler 234 by a further 90.degree. , whereby the signal at output port 234.sup.01 of coupler 234 is the average of two equal-amplitude signals at -90.degree. and 180.degree., which is -135.degree.. Similarly, the odd components at output port 234.sup.02 of coupler 234 together produce a signal, the phase of which is the average of the -90.degree. signal coupled from input port 234.sup.I2 and the -90.degree. signal coupled from input port 234.sup.I1 with an additional 90.degree. phase shift, which once again is the average of two signals at -90.degree. and 180.degree., respectively, which is - 135.degree.. Thus, the odd-mode signals applied to input port 1 of coupler arrangement 228 produce equal amplitude, -135.degree. phase signals at both output ports of coupler 234.
The even mode signals applied to input port 2 of coupling arrangement 228 of FIG. 2c arrive at input port 234.sup.I1 of coupler 234 with 180.degree. phase shift and at input port 234.sup.I2 with 0.degree. phase shift. The even-mode 180.degree. phase signal arriving at input port 234.sup.I1 is coupled to output port 234.sup.01 without additional phase shift, and it is combined by the coupler action with the even-mode 0.degree. signal applied to input port 234.sup.I2, to which a further 90.degree. phase delay is imparted. Thus, the even-mode signal at output port 234.sup.01 of coupler 234 is the sum or combination of two equal-amplitude signals at 180.degree. and -90.degree., which is -135.degree. (indicated in parentheses adjacent to transmission path 256). The even-mode 180.degree. component applied to input port 234.sup.I2 of coupler 234 is provided with an additional 90.degree. phase shift or delay in its coupling to output port 234.sup.02, for a total of -270.degree. or +90.degree., whereby the even frequency signal components at output port 234.sup.02 are at a phase which is the average of the +90.degree. and 0.degree. components, which is +45.degree., as indicated in parentheses adjacent transmission path 258.
The phases of the odd-mode signals originating at input port 1 of coupling arrangement 228 of FIG. 2c are 0.degree., -135.degree., -135.degree., and 180.degree. at transmission lines 248, 256, 258 and 254, respectively. In order to achieve a monotonic horn-to-horn phase progression of 45.degree., the 0.degree. signal on transmission line 248 is phase delayed by 45.degree. in phase shifter 242, to a phase of -45.degree., and the -135.degree. signal on transmission line 256 is phase advanced by 45.degree. in phase shifter 244, to -90.degree.. With only the additions of phase shifters 242 (-45.degree.) and 244 (+45.degree.)(i.e. without phase shifters 246 and 248), the odd-mode signals at the inputs of horns 216b, 216c, 216d and 216e would be placed in the phase -45.degree., -90.degree., -135.degree., -180.degree., respectively, which is the desired phase progression. However, the even-mode signals originating at input port 2 of coupling arrangement 228 of FIG. 2c would then have phases -135.degree. at the output of phase shifter 242, -90.degree. at the output of phase shifter 244, +45.degree. at output port 234.sup.02 of coupler 234, and -90.degree. at output port 233.sup.02 of coupler 233. The progression -135.degree., -90.degree., +45.degree., -90.degree. for the even-mode signals is not the desired monotonic phase progression.
In order to achieve the desired monotonic phase progression of the signals radiated from antennas 216b-216e, output port 233.sup.02 of coupler 233 of FIG. 2c is coupled to phase shifter 246, and output port 234.sup.02 of coupler 234 is coupled to phase shifter 248. With this coupling, and with phase shifts of +45.degree. for phase shifter 246 and -45.degree. for phase shifter 248, the phase progression for the odd-mode signals be comes -45.degree., -90.degree., -135.degree., -180.degree., as indicated adjacent the outputs of phase shifters 242, 244, 246 and 248, respectively, and the corresponding even-mode signals are -135.degree., -90.degree., -45.degree. and 0.degree., respectively. The indicated phases at the outputs of the horns, as indicated in tabular form in FIG. 2c under the heading "Mode" are normalized by the addition of 90.degree.. The normalized progressions are 45.degree., 0.degree., -45.degree., -90.degree. but in mutually opposite directions. Thus, the two phase progressions are monotonic and opposite. The coupling of output port 234.sup.02 of coupler 234 to phase shifter 248, and of output port 233.sup.02 of coupler 233 to phase shifter 246, is accomplished by means of a crossover arrangement illustrated as 240. When the described system is made with hollow waveguide, a crossover such as 240 may be a source of problem. The first aspect of the problem lies in the fact that one waveguide must cross the other in three dimensions, as at crossover region 240 of FIG. 3, which requires the equivalent of two E-plane and two H-plane (total of four) 90.degree. waveguide elbows, each of which contributes an impedance mismatch. Thus, the VWSR of transmission line 254 may be greater than that of transmission line 258. Also, the length of transmission line 254 may be greater than the length of transmission line 248, leading to a need for a compensating phase shift or length of transmission line, which may introduce its own VSWR. Lastly, the crossover is a three dimensional device which is not amenable to ordinary fabrication techniques, but which requires special handling. Its cost may therefore be greater than if the structure were capable of lying in a plane as described below, and where the use of the structure of FIG. 2c is considered for spacecraft use, its weight may be greater than if a simple planar manufacturing technique were available, and its reliability may be inferior.
FIG. 4 is a simplified block diagram of a prior art coupling arrangement 428 which solves some of the abovementioned problems. Elements of coupling arrangement 428 corresponding to those of coupling arrangement 228 of FIG. 2c are designated by the same reference numerals. Coupling arrangement 428 of FIG. 4 differs from coupling arrangement 228 of FIG. 2c in that waveguide crossover 240 is replaced by zero-db hybrid or directional coupler 440. As illustrated in FIG. 4, zero-db coupler 440 includes a first input port 440.sup.I1 which is coupled to output port 234.sup.02 of 3dB hybrid 234, and a second input port 440.sup.I2 which is coupled to output port 233.sup.02 of 3dB coupler 233. Zero-dB coupler 440 also includes a first output port 440.sup.01 coupled to phase shifter 246, and a second output port 440.sup.02 coupled to phase shifter 248. Zero-db hybrid coupler 440 of FIG. 4 is a cascade of two 3dB hybrid couplers, with the output ports of one coupled to the input ports of the other.
FIG. 5 illustrates, in simplified block diagram form, a cascade of two 3dB hybrid or directional couplers 510 and 520 which may be used as zero-dB coupler 440 of FIG. 4. In FIG. 5, first and second input ports 501 and 502 of 3dB hybrid coupler 510 are arranged to receive signal. As illustrated in FIG. 5, only input port 501 receives a signal, with reference amplitude of unity and reference phase angle (1/0.degree.). As is well known, hybrid coupler 510 couples a signal of amplitude .sqroot.2/2 or 0.707, and reference phase (0.707/0.degree.) to an output port 503, and another signal of the same amplitude, but phase delayed by 90.degree. (0.707/-90.degree.) to its output port 504. The two output signals of coupler 510 are applied as input signals to ports 511 and 512 of second hybrid coupler 520. The 0.707/0.degree. input to port 511 produces a signal of 0.5/0.degree. at output port 513 of coupler 520, and a second output of 0.5.degree./-90.degree. at output port 514. The 0.707/-90.degree. signal applied to input port 512 of coupler 520 produces a signal 0.5/180.degree. at output port 513, and a signal 0.5/-90.degree. at output port 514 of coupler 520. Thus, the signal exiting port 513 has two components, each with amplitude 0.5, and with relative phases of 0.degree. and 180.degree.. The components at output port 513 cancel The signal at output port 514, on the other hand, includes two components, each of amplitude 0.5 and phase -90.degree., which sum together to produce signal 1.0/-90.degree.. The energy represented by the canceled components at output port 513 can be viewed as doubling the output power at port 514 from 0.7/-90.degree. to 1.0/0.degree.. It can be seen, therefore, that the cascade of two 3dB hybrid couplers couples the signal from input port 501 to output port 514 with a 90.degree. phase shift. By symmetry, an input applied to input port 502 would appear at output port 513 with a corresponding phase shift. These fixed phase shifts are readily compensated for by appropriate selection of phase shifters 242-248 of FIG. 4.
The use of a zero-dB coupler using two 3-db hybrids solves the crossover and planar manufacture problems, but has been found to be limited in bandwidth.